37 CFR § 2.104 - Contents of opposition.
(a) The opposition must set forth a short and plain statement showing why the opposer believes he, she or it would be damaged by the registration of the opposed mark and state the grounds for opposition. ESTTA requires the opposer to select relevant grounds for opposition. The required accompanying statement supports and explains the grounds.
(b) Oppositions to different applications owned by the same party may be joined in a consolidated opposition when appropriate, but the required fee must be included for each party joined as opposer for each class in which registration is opposed in each application against which the opposition is filed.
(c) Oppositions to applications filed under Section 66(a) of the Act are limited to the goods, services and grounds set forth in the ESTTA cover sheet.